It is known that a latent electrostatic image can be developed with toner particles dispersed in an insulating nonpolar liquid. Such dispersed materials are known as liquid toners or liquid developers. A latent electrostatic image may be produced by providing a photoconductive layer with a uniform electrostatic charge and subsequently discharging the electrostatic charge by exposing it to a modulated beam of radiant energy. Other methods are known for forming latent electrostatic images. For example, one method is providing a carrier with a dielectric surface and transferring a preformed electrostatic charge to the surface. Useful liquid toners comprise a thermoplastic resin and dispersant nonpolar liguid. Generally a suitable colorant is present such as a dye or pigment. The colored toner particles are dispersed in the nonpolar liquid which generally has a high-volume resistivity in excess of 10.sup.9 ohm centimeters, a low dielectric constant below 3.0 and a high vapor pressure. The toner particles are less than 10 .mu.m average by area size. After the latent electrostatic image has been formed, the image is developed by the colored toner particles dispersed in said dispersant nonpolar liquid and the image may subsequently be transferred to a carrier sheet.
Several processes for preparing color liquid toners for electrostatic imaging are known. These include: (A) dispersing in a vessel at an elevated temperature a thermoplastic resin, a dispersant nonpolar liquid having a Kauri-butanol value of less than 30, and optionally a colorant while maintaininq the temperature in the vessel at a temperature sufficient to plasticize and liquify the resin: (B) cooling the dispersion by one of the following:
(1) without stirring to form a gel or solid mass, followed by shredding the gel or solid mass and grinding by means of particulate media in the presence of additional liquid;
(2) with stirring to form a viscous mixture and grinding by means of particulate media in the presence of additional liquid; or
(3) while grinding by means of particulate media to prevent the formation of a gel or solid mass in the presence of additional liquid; and
(C) separating the dispersion of toner particles from the particulate media. The standard media generally used for grinding are balls of stainless steel. Ceramic is another material used as particulate media. It has been found that when stainless steel particulate media are used as the grinding medium in preparing color liquid toners the toners become contaminated, i.e., undergo severe color change. When ceramic type particulate media are used for grinding, the ceramic abrades and contaminates the liquid toner with opaque, relatively large particle pieces of ceramic.
It has been found that the above disadvantage can be overcome and toner particles of excellent color prepared by cooling the toner particle containing dispersion and grinding the particles in the presence of carbon steel particulate media.